Buck Brannaman was brought up in the cowboy tradition of hard work. His skill with horses partly inspired The Horse Whisperer, starring Robert Redford. Photograph: Everett Collection/Rex Features

In a season of big-budget blockbusters, superhero movies and seemingly endless sequels, the surprise hit in America this summer is a documentary about a man who loves horses.

Buck tells the real-life story of Dan "Buck" Brannaman, a cowboy whose almost magical ability to calm unruly horses was an inspiration for the fictional 1998 Robert Redford movie The Horse Whisperer, which also starred Kristin Scott Thomas and a young Scarlett Johansson.

The filmreveals a man whose talent for training horses is rooted in a deeply traumatic childhood at the hands of an abusive father. The film, which won an award at the Sundance film festival, is now playing at more than 200 cinemas across the US: a huge number for a small documentary. "An exceptional slice of Americana about the kind of unsung hero America loves to love," wrote film critic Betsy Sharkey in the Los Angeles Times. Manohla Dargis in the New York Times said that it made for "mesmerising viewing".

Such high praise is all the more remarkable as Buck is director Cindy Meehl's first film. The former fashion designer from the distinctly un-cowboy surroundings of Connecticut met Brannaman nine years ago when she took one of her horses to a travelling "clinic" for problem horses in Pennsylvania.

"It was the first time I had seen him and he showed me that everything I had been doing with horses was wrong. I was very humbled. I realised he was telling me how to speak the horse's language. They were not meant to be slaves," Meehl told the Observer. She became a devotee of his methods and five years later asked if she could make a film about him. "It was a two-minute conversation and he gave me his phone number," she said.

Brannaman's training technique for difficult or troubled horses comes from the so-called "natural horsemanship" movement. Far from "breaking" horses in, Brannaman communicates with them, often by waving small flags. He looks at the world from the horse's point of view, almost acting as a therapist to undo the mistakes or problems of their owners.

In the documentary he is shown with a horse too damaged to be saved. "The horse didn't fail us. We failed him," he says. He currently spends some 40 weeks of the year criss-crossing the country holding horse clinics with the unofficial motto: "Helping horses with people problems."

Meehl reveals that his unique perspective on troubled and damaged horses can in many ways be understood by his own childhood. He grew up in Montana and Idaho, the son of a drunken and abusive father who beat him and his brother. Brannaman was hurt so badly that he would avoid showering with other boys at school in case they saw the wounds on his back. "My dad beat us mercilessly," he says in the film.

Eventually a school sports coach reported his scars and cuts and a local sheriff intervened. Brannaman went to live with a kind but firm Christian couple who fostered children, and it was there he was taught about horses.

It is hard not to believe that Brannaman sees the experience of his own childhood in the panicked and frightened horses that he comes across, using his empathy with abuse to reach out and calm them down.

But Meehl also insists that old-fashioned cowboy values of endless hard work play as big a role. "He says he does not have a special magical gift, but that he just works at it. Then he teaches you that you can, too," Meehl said. She believes that in the end the film is every bit as much about people as it is about horses. "That is why it has the impact it does. People will cry watching this film because he is teaching them that they can be a better person," she said.

Brannaman appeared on the Late Show with David Letterman and Meehl is now planing to take Buck to Europe. In the meantime the debut film-maker who has struck gold first time round is too busy to make concrete plans for another project. "Buck is catching fire. I am working on something new, but I don't have much time at the moment," she said.